Magnetic antenna with low leakage inductance



Se t. 24, 1957 A. A. HEMPHILL ET AL 2,807,802

MAGNETIC ANTENNA WITH LOW LEAKAGE INDUCTANCE Filed March 22. 1954 TRANSMITTING STATION FIG. l5

ALFRED A. HEMPHILL JOHN M. TEWKSBURY INVENTORS MAGNETIC ANTENNA WITH LOW LEAKAGE INDUCTAN CE Alfred Hemphill, Baltimore, and John M. Tewksbury,

Lutherville, Md., assignors to Bendix Aviation Corporation, Towson, Md., a corporation of Delaware Application March 22, 1954, Serial No. 417,613

3 Claims. or. 343-788) This invention relates to magnetic antennas and more particularly to means for reducing the inductance of the.

pickup coil of such an antenna due to the presence of leakage flux paths linking the coil.

This invention has particular application to magnetic antennas arranged for the derivation of directional indications, such for example, as those disclosed in U. S. patent applications, Serial No. 264,717 for Magnetic Antenna System, filed January 2, 1952, by Alfred A. Hemphill, and Serial No. 417,612 for Means for Reducing Leakage Inductance in Magnetic Antennas by Alfred A. Hemphill and John M. Tewksbury, filed March 22, 1954.

In antennas of this type, using elongated collector bars of high permeability arranged radially about a coil wound on a highly permeable rotatable core, as the coil is rotated past the pole pieces the leakage flux through pole pieces of collector bars which are in positions of minimum pickup increases the inductance of the coil. When these flux paths pass through only one pole piece the inductance of the coil is increased to a greater extent than when the flux paths include portions .of more than one pole piece and the air gap between'them. This differing inductance affects the pickup, introducing distortion into the directional indication and tending to mask the null indication. When the antenna is being used for reception, without reference to the indication of direction, the effect of the leakage flux to reduce the pickup is still undesirable.

It is an object of this invention to provide a means for effectively reducing the leakage inductance and the variation thereof occurring under the above circumstances.

It is a further object of the invention to provide this reduction by means which is inexpensive and simple.

It is another object of the invention to provide means of this type which does not introduce significant errors into the system.

These and other objects and advantages of the invention may be realized by providing for each collector bar two pole pieces rather than one, the two pieces having the same configuration as though a single pole piece symmetrical about a vertical plane of symmetry had been divided along that plane into two'equal portions.

In the drawing:

Fig. 1 is a plan view of a magnetic antenna system showing the linkage of leakage flux with the coil of the rotor and the pole pieces, under a certain set of conditions;

Fig. 2 is a plan view of the antenna of Fig. 1 with the rotor rotated to a different position, showing the linkage of the leakage flux under this changed set of conditions; and

Fig. 3 is a plan view of a magnetic antenna embodying the invention.

Referring now more particularly to the drawing, there is shown in Fig. 1 a magnetic antenna comprising a centrally located rotor, made up of a disk-shaped core 1 of highly permeable material, having wound about it a 2,807,802 Patented Sept. 24, 1957 pickup coil 2. Positioned symmetrically about the rotor are a plurality of pole pieces 3 also formed of highly permeable material. The faces of these pole pieces which lie adjacent to the rotor are arcuate in form, the curvature being concentric with that of the lateral periphery of the rotor. Air gaps 9 separate adjacent pole pieces.

Associated with each of the pole pieces is a respective collector bar 4, 5, 6 or 7 extending radially from the center of the rotor. These bars are constructed of high- 1y permeable material. The inner ends of the bars are arranged in overlapping relationship with the outer ends of the pole pieces with which they are associated, so that parallel surfaces of the bar and its associated pole piece are either in contacting relationship or in very close proximity to each other, thus providing a path of low reluctan'ce passing through the bar and its pole piece.

" about from the null position.

rotation the signal will decrease until another null point In the use of an antenna arrangement such as is illustrated in Fig. 1, for direction finding, the rotor is rotated with respect to the collector bars to find a position of minimum signal pickup, or the so-called null point of the signal, which defines the direction of the transmitting sta- .tion. In positions other than this the rotor will pick up a signal which will gradually increase in intensity as the rotor is turned from the null position until it passes through a broad region of substantially maximum signal, Upon continued is reached, at substantially from the first.

. While the above description of the operation of the antenna indicates the presence of a sharp null point in the sensitivity pattern thereof, there is present in'practice an effect which tends to obscure the null and to produce a distorted output.

In the illustration of Fig. 1, let it be assumed that the collector bar 4 is pointed in the direction of the transmitting station, as indicated by the arrow accompanying the figure. The flux set up in the collector bars by the magnetic field of the wave advancing from the station will be a maximum through bars 5 and 7 and a minimum through bars 4 and 6. With the orientation of the core and coils shown, the flux passing through the rotor should cause a maximum current flow through the coil and hence, a maximum pickup.

However, there will be a certain amount of leakage flux, as indicated by the closed lines 8, flowing through the core 1 and the pole pieces 3 associated with collector bars 4 and 6. This flux will increase the inductance of the coil. The change of inductance in the coil will impair the match between the coil and the tuned circuit (not shown) normally coupled to the coil in such systems for applying the signal to the remainder of the system. This impairment of the coupling will reduce the maximum pickup of the system, tending to obscure the null by rendering its sides less steep. The change in inductance also results in the detuning of the above mentioned tuned circuit, causing bearing error because of the non-symmetry of the two slopes of the null.

It should be noted that in the orientation shown in Fig. l and described above the lines of leakage flux 8 do not traverse any of the air gaps 9 between the pole pieces so that the paths are of a low reluctance nature.

Assume now that the rotor is rotated in a clockwise direction through an angle of 45 as indicated in Fig. 2. Now it will be seen that the lines of leakage flux 8 traverse one of the air gaps 9. As a consequence, the a paths pass. through anair gap and rising when the paths do not include such a gap, The undulating patternof distortion which is applied to the pattern of response as a result serves to obscure and displace the null indication and to lower the amplitude of themaximum pickup. The

latter, defecttimparticularly obiwtionable when: the system;

is being used for communication purposes.

In. a wrdance with the instant; invention the. defect: described above is reduced to such .antextent astoibe. iectionable .by a. construction such. as illustrated in.

Fig; .3, in which, instead of. the unitary pole piece 3 proyidedqfor each collector .bar in the system of Fig. 1, ere. is provided. apair; of. polepieces 10 and: 11, lying side-by-side. and. separated; by a uniform air gap. 12.} hese polepieces are of astructure similar. to that which.

o ldsresulttifuone ofthe polei pieces 3 were split into imp rts along its verticalrplane of symmetry and;those partssmoved apart to provide the :ain gap 12 therebetween.

The. efiecti of theconstruction illustrated in Fig. 31s to providotan -additionalair gap in the OOlltCliDfucilCh POIQXPW. 3, so that whenuthe. lines of leakage fluicare position d asshowninfiig; 1 theifLp -ths will include an an 83D and their, reluctance will;be increased] to substantially the ame value .as the. .paths for. the orientation of ile, 2,. The peaks. of the. distortion pattern. are thus; removed and thepattern as awhole. mademore uniform.

The ndulations which remain. in the pattern; are. small; enough so that the error, remaining is of; a1 valuelow enough to .be. unobjectionable. Gare should be: main tained. toinsure; that the pole pieces. 10. and 11: are as nearly identical, as possible, since any. departurefrorn identity willjntroduce. distortion into the. output. The amount. of; this distortion is small, but. with care it. can be completely eliminated.

What is claimed-is: 1

. 1., A. magnetic antenna :comprising. a rotor including a pickup .coil wound upon a rotatable highly, permeable ore; aplurality: ft collector bars formedlofrhighly permeable material symmetrically disposed aboutsaid rotor, saidbarsextending radiallyfromthc center of said core and lying in a common plane, and, a pair of pole pieces formed of highly permeable material. disposed between each ofgsaid collectqrbars and said rotor. and in abutting relationship with said her, whereby any air, gap exists.

between. said. bar and saidpole. pieccs,.said polepieces ofsaid pair presenting substantially. equal. reluctances to formed of highly permeable material disposed between each of said collector bars and said rotor and in abutting relationship with said bar, whereby an air gap exists between said her and said .pole pieces, said pole pieces of said pair presenting substantially equal reluctances to the flow of magnetic flux between said rotor and said bar, said pole pieces of said pair being completely separated by an air gap of uniform cross-section and being symmetrical with respect to a plane normalto saidcommon plane and containing the longitudinal axis of saidbar.

3. A magnetic antenna comprising a rotor including a pickup=coil wound upon a rotatable, highly permeable core; a plurality of collector bars formed of highlypermeableimaterial symmetricallyr disposed about said rotor, said bars extending radially from the center of said core and lying in a common plane, and a pair of pole pieces formed of highly permeable material disposed between eachiof said collector bars and saidtrotor and in abutting relationship with said bar, whereby an air gap exists between said bar and said pole pieces, said pole pieces of said pair presenting substantially equal reluctances to the-flow of magnetic flux between said rotor and said bar, said pole pieces of said pair being completely separated by. anair gap of uniform crosssection and being symmetrical with respect to a plane normal to said common plane. and containingthe longitudinal axis of said bar, each pole piece of said pair being spaced from one of'the pole piecesof an adjacent pair by an air gap having substantially the same reluctance as the said .air gap separating the pole pieces of said pair.

ReterencestCited inthe file of this patent FOREIGN PATENTS 874,946 France Aug. 31, 1942 

